Ordinary cells include a bottomed cylindrical metal cell case that is open only at an end thereof and operates as negative electrode, and a sealing member having a metal electrode plate operating as positive electrode. For connecting a plurality of cells in series, the electrode plate of each of the cells and the cell case of the adjacent cell are electrically and mechanically connected to related respective spots of a single connector by spot welding.
During the spot welding operation, no pinching pressure is applied to the parts being welded by welding electrodes and a large electric current flows between the welding electrodes as surface current running along the surfaces of the connector and the electrode plate. A connector having a large thickness or showing a low specific resistance cannot be used because such a connector makes the welding operation highly difficult. As a result, the electric resistance of the connection between the cells becomes large to give rise to a large loss at the connection of the cells where a large electric current flows. Additionally, the strength of the mechanical link of the cells is inevitably weak because of the weak mechanical strength of the connector itself.
In view of the above mentioned circumstances, the inventors of the present invention has already proposed cell connecting structures showing a reduced connection resistance and an enhanced connection strength as respectively shown in FIGS. 13 and 14 by improving the structure of known connectors and the conventional welding method. In the case of a proposed cell connecting structure (see Japanese Patent Laid-Open Publication No. 2000-149907) as shown in FIG. 13, a connecting electrode section 3 is formed on the electrode plate 7 of the sealing member 2 for closing the opening of a cell case 1, the connecting electrode section projecting axially outwardly. Then, the connecting electrode section 3 of the cell B1 of a pair of adjacent cells B1 and B2, is welded to the bottom surface 4 of the cell case 1 of the other cell, namely the cell B2, by projection welding by way of projections (not shown) formed on the connecting electrode section 3, and the connecting electrode section 3 and the bottom surface 4 are directly welded on each other at nuggets 8, thereby connect the cells B1 and B2 in series.
With a proposed cell connecting structure (see Japanese Patent Laid-Open Publication No. Hei. 10-106533) as shown in FIG. 14, a connector 9 is employed to connect a pair of adjacent cells B3 and B4. The connector 9 includes a flat bottom section 11 to be held in contact with the electrode plate 10 of one of the cells, namely the cell B3, operating as positive electrode, and a cylindrical section 12 to be arranged like a sleeve around the cell case 1 of the other cell, namely the cell B4, operating as negative electrode. The cylindrical section 12 is provided with a plurality of circularly arranged projections 13 to be used for projection welding, while the bottom section 11 is also provided with a plurality of circularly arranged projections 14 to be used for projection welding. Note that FIG. 14 shows the arrangement before the welding operation. The connector 9 and the cell B3 are welded one on the other as they are welded to each other by projection welding by way of the projections 14 on the bottom section 11, while the connector 9 and the cell B4 are welded on each other as they are welded to each other by projection welding by way of the projections 13 on the inner surface of the cylindrical section 12 on the cell case 1.
The above described cell connecting structures are so designed as to bond the adjacent cells B1, B2, B3, and B4 respectively by projection welding to establish an electric connection there. With the projection welding, the two components to be welded are brought into contact with each other by way of the small areas of the projections 13 and 14 so that the electric current flowing through them is concentrated in the small contact areas to generate heat there because of the high electric resistance and melt the projections for welding. Consequently, unlike spot welding, these arrangements do not require high pinching pressure.
However, the above described cell connecting structures are still accompanied by problems that needs to be dissolved before marketing them, and they need improvements. In the case of the cell connecting structure of FIG. 13, the electric plate 7 is formed by press molding so that the projections on the electrode plate 7 to be used for projection welding are inevitably made thin. Then, as the electric current flowing through the components to be welded is concentrated in the thin projections during the projection welding process, the thin projections may become molten excessively to give rise to cracks there so that consequently the electrolyte contained in the cells may leak out through the cracks.
On the other hand, the cell connecting structure shown in FIG. 14 requires that the bottom section 11 of the connector 9 is welded on the electrode plate 10 of the cell B3 by projection welding and subsequently the cylindrical section 12 of the connector 9 is welded on the cell case 1 of the cell B4 also by projection welding. The latter projection welding operation needs to be conducted in a very unstable condition.
More particularly, the cell case 1 of the cell B4 is sleeved by the cylindrical section 12 of the connector 9 so that the plurality of projections 13 for projection welding projecting from the inner surface of the cylindrical section 12 are made to abut the outer peripheral surface of the cell case 1. Then, under this condition, the positive and negative welding electrodes 17 and 18 are made to abut the outer peripheral surface of the cell case 1 and the cylindrical section 12 respectively and an electric current is made to flow from the cell case 1 to the cylindrical section 12 by way of the projections 13 for projection welding.
Thus, it is not easy to successfully carry out the projection welding operation because the welding electrode 18 needs to abut the outer surface of the cylindrical section 12 that is held in an unstable state. In other words the adjacent cells B3 and B4 may not necessarily be bonded to each other in a good condition to establish an electric connection between them.
Additionally, the projections 13 of the connector 9 need to have accurately a same height so that they are made to abut the outer peripheral surface of the cell case 1 under even and uniform pressure. Still additionally, although not shown in FIG. 14, the cylindrical section 12 is provided with a plurality of slits in order to provide it with resiliency so that it applies pressure onto the cell case 1 when it is put on the cell case 1. This means that the connector 9 has a complex profile and requires precision processes for forming it in good shape.
In the light of the above mentioned problems, an object of the present invention is to provide a method and a structure for connecting cells that allow a projection welding operation of bonding and electrically connecting two adjacent cells to be carried out easily and reliably and secures a good welding quality for the projection welding operation without giving rise to any problems such as solution leakage.